In thermal power stations, the feed water is heated stepwise in preheaters before it enters the steam generator. These preheaters can be either the vertical type or the horizontal type. When superheated steam is introduced into the feed water preheater, a part of the superheat can be thermodynamically utilised in a desuperheater, if the steam is sufficiently superheated. The steam is introduced into the desuperheater through a branch directed to the tube bundles and is passed around the tube bundles in a counter-current fashion and thus heats the feed water flowing in the tubes by convection. In desuperheaters of the horizontal type, the bleed steam is passed, depending on the degree of superheat, at high velocity in the axial direction of the preheater through one or more chambers arranged in the desuperheater and then flows into the condensation zone of the preheater. Due to the flow losses suffered by the steam while passing through the desuperheater until it leaves the latter, the steam pressure in the condensation part of the preheater is substantially lower than at the desuperheater inlet.
In the known designs of horizontal preheaters, the steam outlet ports are arranged on the end face of the desuperheater which faces the condensation zone or are arranged on the last desuperheater support plate. At this point, some of the preheated tubes are not supported, and the steam flows through the outlet ports directly onto the condensation tubes and along them.
In this way, a crossflow results between the steam leaving the desuperheater and the condensate dropping off from the condensation tubes, whereby, in particular at high steam velocities, the condensate is entrained by the steam and whirled against the condensation tubes. This action can cause erosion/corrosion damage to the condensation tubes.